The present invention relates to the field of electric heaters, particularly ceramic heaters as commonly used in compression type ignition engines.
The use of heaters in operating compression type ignition or diesel engines is well known. These heaters, commonly referred to as glow plugs, are installed in the engine such that a portion of the heater extends into the combustion cylinder, thereby transferring heat to the air or fuel/air mixture contained in the cylinder.
Historically this transfer of heat has been used to ignite the fuel in the starting of engines and this is still currently done in some applications. Before starting the engine, the heater is manually activated. Once the heater reaches a predetermined temperature, the engine can be started and the heater can be shut off. Engine start-up is thereby greatly facilitated, particularly in cold climates. Continuous heating also improves the efficiency of combustion, however, and consequently efforts have been made to increase the duration of time that the heater remains active following engine start-up. These efforts have resulted in a controlled xe2x80x9cafter-glowxe2x80x9d application in which the heater would remain active until the engine reached normal operating temperature. More recently this has been further extended to achieve prolonged or even continuous heater operation.
Extending the activation period of heaters has not been without difficulty. One major concern is the risk of overheating, namely when the engine warms up, the cooling effect on the heater is greatly reduced. An activated heater therefore will continue to build up heat, incurring the risk of reaching a temperature exceeding that which the material used to construct heater can withstand. Related to this problem is the fact that temperature conditions in the combustion chamber can fluctuate during normal operation because of, for example, changes in load experienced by the engine. In what is known as xe2x80x9chigh rpm, low loadxe2x80x9d conditions, the ratio of air to fuel drawn into the combustion chamber is much higher than required for efficient stoichiometric combustion, resulting in a significant cooling effect. Under these conditions, heaters operating continuously should increase output to compensate for the cooling effect. Thus temperature regulation against overheating and overcooling is required in heaters which operate in prolonged or continuous use applications.
The risk of overheating was particularly acute in earlier heaters constructed from metal materials. Since then ceramic has become a much more popular choice because it is able to withstand higher temperatures. Ceramic heaters can heat up more quickly, maintain a higher operating temperature, and are more resistant to corrosive elements than metal heaters. The ceramic materials selected also possess a Positive Temperature Coefficient (PTC) of electrical resistance wherein an increase in temperature results in a corresponding increase in electrical resistance. As the temperature of a PTC material increases, the resistance to the flow of the electrical current also increases. At high temperature the resistance increases so that the heater draws less current, thereby protecting itself against overheating.
There are a variety of existing heater designs which incorporate the use of ceramic materials. In one such design a filament made from a metal such as tungsten is imbedded in a ceramic cylinder. This design is described in, for example, U.S. Pat. No. 4,357,526 to Yamamoto et al. Although this design captures some of the benefits associated with ceramic materials, it is weak in terms of the integrity of the electrical circuit at high temperatures. Efficient heating depends on a reliable electrical connection between the filament and the surrounding ceramic, but metal-to-ceramic connections in which the ceramic acts as the heating element are difficult to maintain, due in part to embrittlement and ultimately decomposition of the metal. In addition, the electrical current capability of the heater is limited by the relatively small diameter of the filament. A larger filament would increase stresses on the assembly due to the differences in thermal expansion properties of ceramic and metal.
Improved ceramic heater designs exist in which the heater is constructed from ceramic materials alone, although these types of heaters also suffer from a number of disadvantages. For example, the all-ceramic heater element disclosed in U.S. Pat. No. 6,084,212 to Leigh suffers from various disadvantages associated with what is typically known in ceramics as micro-cracking. Ceramic heaters generally undergo severe thermal stresses due to rapid heating and cooling effects in an engine. Since Leigh substantially narrowed heater tip, micro-cracks which originate from the surface, grow slowly through the ceramic materials causing the narrowed tip to break off. Further, the overly thin layers utilized within the heater are prone to failure at an early stage of crack propagation since the crack only has to run a relatively short distance before becoming problematic. Due to the narrowed tip, the glow plug heater is more prone to thermal cycling because of a reduced thermal mass, which itself can rapidly accelerate stress induced cracking. Finally, in order to provide sufficient heating volume, a relatively large diameter base portion is required. A large-based heater is not always feasible due to the space allowances associated with installation hole in an engine.
The ceramic heater designs comprising separate heater and regulator elements typically use materials with different PTC characteristics for the two elements to improve the self-regulating capabilities of the heater. By selecting a ceramic for the regulator with a higher PTC than that of the heater element, a more controlled temperature profile can, in theory, be obtained. Practically, however, there are some adverse effects resulting from this design. Any temperature fluctuations in the combustion chamber must first be transmitted through the ceramic heater element before being sensed by the regulator element. This results in a delayed response which in some cases can cause the regulator to control the current flow in a manner which is opposite to what is immediately required at the end of the heater.
An additional drawback of the separate regulator and heater designs is that they typically require that the heater to have a tip with a reduced diameter. This characteristic can be observed in heater designs disclosed in, for example, U.S. Pat. No. 4,682,008 to Masaka, where the tip of the heater is narrowed in order to generate greater resistance, and accordingly a concentrated heat zone. If this is not done, the heater would generate heat along the entire length of the element and thereby consume an excessive amount of power. However, narrowing the tip reduces the surface area and overall volume of the heater element in the combustion chamber. This in turn reduces the rate of heat transfer from the heater to the air around it, which reduces the overall performance of the heater. Alternatively, an enlarged base may be employed in the above tapered heater design, but that is undesirable in the case of most engines where a larger installation hole is prohibited.
These drawbacks are overcome to some extent in heater designs comprised of a single ceramic element that provides both the heating and regulatory functions. However, typical designs still require a narrower diameter at the tip and are subject to the drawbacks associated with a narrowed tip as discussed above. Existing single element designs also contain a point of contact between the ceramic heater element and a metal member. This combination of materials positioned adjacent to each other presents significant problems. As current flows from one material to the other, the connection degrades and eventually leads to failure of the heater. In order to counteract this problem and achieve an acceptable useful life, these heaters are operated at lower power levels, which compromises the performance of the heater.
The present invention provides a heater having a tip, said heater comprising:
(a) an electrode;
(b) an insulative layer disposed over the outer surface of said electrode;
(c) a resistive layer disposed over said insulative layer such that a substantial portion of the volume of said resistive layer is disposed in close proximity to the tip of the heater; and
(d) a conductive layer which is disposed over said insulative layer.
In another aspect, the present invention provides a heater having a tip, said heater comprising:
(a) an electrode comprising a first portion having a resistance that varies with temperature, a substantial portion of the volume of said first portion being disposed in close proximity to the tip of the heater;
(b) an insulative layer disposed over the surface of said electrode;
(c) a resistive layer disposed over said insulative layer; and
(d) a conductive layer which is disposed over said insulative layer.
In another aspect, the present invention provides a ceramic heater comprising:
(a) a resistive heater portion; and
(b) a regulatory portion coupled to said heater portion, said regulatory portion having a negative temperature coefficient of resistance for regulating the power in the heater.
In another aspect, the present invention provides a method of fabricating a heater having a tip, said method comprising the steps of:
(a) forming an electrode;
(b) forming an insulative layer and positioning it over the electrode;
(c) forming a resistive layer and positioning it over the insulative layer such that a substantial portion of the volume of the resistive layer is disposed at the tip of the heater;
(d) forming a conductive layer and positioning it over the insulative layer; and
(e) slip casting the electrode, insulative layer, the resistive layer and the conductive layer to form a green body.